Development of biosensors using enzymes that react specifically on specific substrates are actively under way regardless of the industrial field. A typical example of biosensors is the glucose sensor, which is mainly used in the medical field.
The glucose sensor serves to build a reaction system that contains an enzyme and an electron transmitter, and when using this glucose sensor, the glucose is quantitativley determined using, for example, the amperometric technique. Glucose oxidase (GOD) or glucose dehydrogenase (GDH) is used as the enzyme (Japanese Patent Application Laid-open No. 2002-65778).
Since GOD has high substrate specificity towards glucose and has excellent thermostability, massive production of the enzyme is possible, such that it has the advantage that the manufacturing cost is inexpensive compared to other enzymes. However, systems that use GOD have the problem that they are easily influenced by the oxygen that is dissolved in the measurement sample, such that the dissolved oxygen exerts an effect on the measurement results.
On the other hand, systems that use GDH are not easily influenced by the oxygen that is dissolved in the measurement sample. Therefore, systems that use GDH can measure glucose concentration with good accuracy even when carrying out measurements in an environment where oxygen partial pressure is low, or when measuring high concentration samples that require large quantities of oxygen. However, GDH has the problems that its thermostability is bad and its substrate specificity is worse than GOD.
From such circumstances, an enzyme that complements the shortcomings of both GOD and GDH was sought. SODE, who is one of the present inventors, isolated a new strain (Burkholderia cepatia KS1 strain) from a soil in the neighborhood of a thermal spring and obtained a novel GDH from this strain as disclosed in International Patent No. WO02/36779. This GDH consisted of α, β and γ subunits (hereinafter referred to as “CyGDH”), its reaction rate with the electron transmitter was high, and had no problem in regards to heat resistance, such that it was suitable as an enzyme for use in a glucose sensor.
However, since productivity of CyGDH was bad in the KS1 strain, massive production of CyGDH by the KS1 strain was difficult when considering an industrial application. When the present inventors therefore introduced the DNA coding for the α, β and γ subunits into Escherichia coli and expressed them, GDH was efficiently produced. However, this GDH consisted of the α, and γ subunits, which was missing the β subunit (hereinafter referred to as “αGDH”). As described, the total of α, β and γ subunits could not be expressed by the process of transforming Escherichia coli. 
In addition, when the present inventors examined the characteristics of αGDH, αGDH was found to have a slower reaction rate with the electron transmitter compared to CyGDH, but higher heat resistance than CyGDH, and a smaller Km for glucose. That is to say, αGDH was identified to be as useful as CyGDH, as an enzyme for use in a glucose sensor.
In prior art, to prepare 2 species of enzyme that are useful in this way, it was necessary to carry out separately each of the acquisition of expression strain, culture and purification, which was disadvantageous from the perspective of manufacturing costs and efficiency.